Additives have been commonly used to try to improve the performance of lubricating oils for gasoline and diesel engines. Additives, or additive packages, may be used for a number of purposes, such as to improve detergency, reduce engine wear, stabilize lubricating oils against heat and oxidation, reduce oil consumption, inhibit corrosion and reduce friction loss. “Dispersants” are used to maintain insoluble materials formed by oxidation and other mechanisms during the use of the oil within the oil, to prevent sludge flocculation and the precipitation of these insoluble materials. Other functions of the dispersant include the prevention of soot particle agglomeration, providing sooted oil rheology control, and the prevention of deposit formation on the engine pistons.
Substantially all dispersants in use today are reaction products of (1) a polyalkenyl-substituted mono- or dicarboxylic acid, anhydride or ester (e.g., polyisobutenyl succinic anhydride or “PIBSA”), also commonly referred to as a carboxylic acid acylating agent; and (2) a nucleophilic reactant (e.g., an amine, alcohol, amino alcohol or polyol). More specifically, substantially all dispersants used commercially are succinimide dispersants; the reaction product of polyisobutenyl succinic anhydride and an amine, particularly a polyamine.
One approach to enhanced dispersant performance has been to increase the average number of polar head groups on the hydrophobic polymer chains of the dispersant. This enables more interaction with incipient sludge and deposit precursors or soot particles. Such enhanced interaction can be expected to result in improved performance provided that the hydrophobic polymer chain is large enough to maintain solubility in the oil while carrying the increased polar load. A method of increasing the average number of polar head groups per hydrophobic polymer chain involves increasing the succination ratio, or functionality (F) of succinimide dispersants. Such a method is described in U.S. Pat. No. 4,232,435, which discloses the use of the chlorine-assisted maleation process to yield high functionality dispersants with significant chain extension. Chain extension occurs when the polyfunctional succinic groups condense with polyfunctional amine groups to significantly increase the size of the dispersant). These chain-extended dispersants provided elevated thickening efficiency. However, the large size of the chain-extended dispersants actually caused deposit control to deteriorate, rather than improve and also caused interactions with other polar additives, specifically detergent colloids, to increase dramatically. Blending these dispersants with overbased detergents often resulted in gelation, requiring special blending procedures to prepare additive concentrates. Hence, succinimide dispersants with moderate functionality levels have been considered more desirable for both performance reasons (improved deposit control) and for making additive concentrates in combination with overbased detergents (see U.S. Pat. Nos. 6,743,757; 6,734,148; and 6,127,321).
The polyalkenyl-substituted mono- or dicarboxylic acid, anhydride or ester from which succinimide dispersants are derived can be prepared using either a halogen (e.g., chlorine) assisted alkylation process, and those prepared using a halogen-free thermal, or “ene” reaction process. When using “conventional” Lewis acid catalyzed polyalkenes, high functionality carboxylic acid acylating agents, particularly polyisobutylene succinic anhydride (“PIBSA”) cannot be provided via the halogen-free thermal reaction process. However, PIBSA having a higher functionality (e.g., a functionality of greater than 1.3) may be prepared via the thermal process using a polyalkene (particularly polyisobutylene) having a high terminal vinylidene content (greater than 65%, such as greater than 70%, 80% or 85%). Processes for producing high terminal vinylidene content polyisobutylene products (referred to as highly reactive polyisobutylene or “HR-PIB”) are described, for example, in U.S. Pat. No. 4,152,499, and HR-PIB products are commercially available from TPC, or from BASF under the trade name Glissopal™. PIBSA having a functionality of greater than 1.3 prepared via the thermal process using polyisobutylene having a high terminal vinylidene content are described, together with succinimide dispersants derived therefrom, are described in EP 0 355 895. Moderate functionality dispersants derived from PIBSA produced via the thermal process using HR-PIB (“thermal dispersant”) were shown to provide improved piston cleanliness performance compared to corresponding dispersants derived from PIBSA prepared using the chlorine assisted alkylation process (“chloro dispersant”) in U.S. Pat. No. 6,743,757. The use of a mixture of a relatively high molecular weight chloro dispersant and 20 to 40 by mass, based on the total mass of dispersant, of a relatively low molecular weight thermal dispersant is described in U.S. Pat. No. 7,615,521 as reducing the increase in viscosity of an additive package containing an overbased sulfonate detergent during storage compared to when the high molecular weight chloro dispersant is used as the sole dispersant.
The present invention is directed to additive concentrates containing very high functionality dispersants that do not cause high levels of viscosity increase when blended with overbased detergent colloids yielding additive concentrates that remain fluid, and provide improved piston deposit control performance. While not wishing to be bound by any specific theory, it is believed that high functionality dispersants compatible with overbased detergent colloids can be provided by clustering the succinic functional groups at the terminal end of the polymer chain of the dispersant while simultaneously minimizing the extent of “remote” functionalization, wherein some succinic functional groups are placed at a distance from the terminal end of the polymer chain. A remote architecture appears to increase the size of the dispersant and the occurrence of bridging between two overbased detergent colloid particles in a concentrate, which results in gelation or very high concentrate viscosities. In contrast, the clustered architecture allows for an increased number of polar head groups per polymer chain, while simultaneously constraining viscosity growth due to chain extension to a modest level, resulting in improved deposit inhibition and enabling blending of fluid additive concentrates containing both the dispersant and overbased colloidal detergents. This clustered architecture can be produced with a polymer having a high terminal vinylidene content using an ene or thermal maleation process, rather than a chlorine-assisted maleation process, to incorporate the succinic functionality.